1. Field of the Invention
The invention relates to a lamp driving circuit and a control method thereof, more particularly to a lamp driving circuit adapted for a discharge lamp and a control method thereof.
2. Description of the Related Art
In recent years, as discharge lamps, such as hot cathode fluorescent lamps, cold cathode fluorescent lamps, external electrode fluorescent lamps, neon lamps, etc., become widely used in backlight systems of liquid crystal display devices, advertisement displaying devices, and general lighting devices, etc., it is increasingly important for lamp driving circuits that convert direct-current (DC) power to alternating-current (AC) power for driving the discharge lamps to be compact and highly efficient.
As shown in FIG. 1, a conventional drive circuit is adapted for driving at least one discharge lamp 74. When the conventional drive circuit is adapted for driving a plurality of the discharge lamps 74, the discharge lamps 74 need to be connected in parallel to each other. The following description is presented using an example where the conventional drive circuit is adapted for driving a single discharge lamp 74.
The conventional discharge lamp includes a step-up transformer 71, a detector 72, and a controller 73.
The step-up transformer 71 includes a primary winding 711 and a secondary winding 712. The secondary winding 712 is adapted to be coupled electrically to the discharge lamp 74, and is adapted to cooperate with the discharge lamp 74 to form a tank circuit that generates a tank current. The tank circuit is composed of leakage inductance 716 of the secondary winding 712, distributed capacitance of the secondary winding 712, stray capacitance around the discharge lamp 74, and a suitably added auxiliary capacitance 75.
Resonance frequency of the tank circuit can be calculated using the equation below:
      f    r    =      1          2      ⁢                          ⁢      π      ⁢                                    L            s                    ⁡                      (                                          C                w                            +                              C                a                            +                              C                s                                      )                              
where fr denotes the resonance frequency of the tank circuit, Ls denotes the leakage inductance 716 of the secondary winding 712, Cw denotes the distributed capacitance of the secondary winding 712, Cs denotes the stray capacitance around the discharge lamp 74, and Ca denotes the auxiliary capacitance 75.
There are two conditions for increasing the efficiency of the conventional drive circuit, one of which is for a phase difference between a voltage and a current of the primary winding 711 of the step-up transformer 71 to approach zero, and the other one of which is to drive the step-up transformer 71 near or below the resonance frequency.
The detector 72 is for detecting phase of the tank current, current magnitude of the discharge lamp 74, and voltage magnitude of the secondary winding 712 of the step-up transformer 71, and outputs a first detecting signal corresponding to the phase of the tank current, a second detecting signal corresponding to the current magnitude of the discharge lamp 74, and a third detecting signal corresponding to the voltage magnitude of the secondary winding 712.
The detector 72 utilizes a Zener diode 721, which is connected in series to the auxiliary capacitance 75, and whose anode is grounded, to detect the phase of the tank current, so as to obtain the first detecting signal. With reference to FIG. 2, a set of example waveforms are shown, where waveform 801 represents the tank current, and waveform 802 represents the first detecting signal, the horizontal axis denoting a time axis (t).
Referring back to FIG. 1, the controller 73 is coupled electrically to the detector 72 and the primary winding 711 of the step-up transformer 71, and includes a switching unit 731, an analog-to-digital converting unit 732, an oscillator unit 733, a processing unit 734, a burst unit 735, and a waveform generating unit 736.
The switching unit 731 is coupled electrically to the primary winding 711 of the step-up transformer 71, and to the waveform generating unit 736 for receiving a control signal therefrom. The switching unit 731 further receives a direct-current (DC) power signal from a DC power source, and generates a drive signal for driving the step-up transformer 71 from the DC power signal based on the control signal. The drive signal is a periodic alternating-current (AC) signal.
The switching unit 731 is a full bridge circuit, and includes four switches, namely a first switch 761, a second switch 762, a third switch 763, and a fourth switch 764. The first switch 761 is coupled electrically between a first end of the primary winding 711 and ground, the second switch 762 is coupled electrically between the first end of the primary winding 711 and the DC power source, the third switch 763 is coupled electrically between a second end of the primary winding 711 and ground, and the fourth switch 764 is coupled electrically between the second end of the primary winding 711 and the DC power source. The control signal includes a set of control sub-signals that respectively correspond to the first to fourth switches 761˜764.
Waveforms of the control sub-signals for the first to fourth switches 761˜764 of the switching unit 731, of the drive signal provided to the primary winding 711 of the step-up transformer 71, and of current flowing through the primary winding 711 in a situation where a phase difference between the current flowing through the primary winding 711 and voltage across the primary winding 711 is zero, are shown in FIG. 3, the horizontal axis denoting a time axis (t). Waveforms 811˜814 respectively represent the control sub-signals for the first to fourth switches 761˜764, waveform 815 represents the drive signal, and waveform 816 represents the current flowing through the primary winding 711, where Tdrive denotes a period of the drive signal, Tduty denotes a duration of a positive pulse or a negative pulse of the drive signal, and Toverlap denotes a discharge duration to release energy stored by the primary winding 711. It should be noted herein that since Toverlap is much smaller than Tdrive, Toverlap is enlarged in FIG. 3 for illustrative purposes.
High voltage levels of the waveforms 811˜814 respectively represent closing (i.e., a conducting state) of the first to fourth switches 761˜764, while low voltage levels of the waveforms 811˜814 respectively represent opening (i.e., a non-conducting state) of the first to fourth switches 761˜764. The positive and negative pulses of the drive signal have an absolute voltage magnitude equal to that of the DC power signal. A positive peak of the current flowing through the primary winding 711 of the step-up transformer 71 corresponds in time to a center point of the positive pulse of the drive signal, while a negative peak of the current flowing through the primary winding 711 corresponds in time to a center point of the negative pulse of the drive signal.
The phase difference between the current flowing through the primary winding 711 and the voltage across the primary winding 711 can be adjusted by adjusting Tdrive. Current flowing through the discharge lamp 74 can be adjusted by adjusting Tduty, where Tduty is adjusted by varying duration of the positive/negative pulse of the drive signal in equal-amounts to the left and right with respect to a center of the positive/negative pulse. Since the first switch 761 and the third switch 763 are disposed in the conducting state simultaneously for a period of time (i.e., during Toverlap), both the first and second ends of the primary winding 711 are grounded simultaneously, and energy stored by the primary winding 711 can be discharged to facilitate reversal of the direction of the current flowing through the primary winding 711. Toverlap needs to be large enough for the primary winding 711 to be sufficiently discharged. Discharging of the primary winding 711 can also be achieved by closing the second switch 762 and the fourth switch 764 simultaneously such that the two ends of the primary winding 711 are coupled electrically and simultaneously to the DC power source.
A duty ratio of the drive signal is calculated as follows:
      R    duty    =                    2        ·                  T          duty                            ·                  T          drive                      ×    100    ⁢    %  
where Rduty denotes the duty ratio of the drive signal, Tdrive denotes the period of the drive signal, and Tduty denotes the duration of the positive pulse or the negative pulse of the drive signal.
The larger the duty ratio of the drive signal, the larger will be the current flowing through the discharge lamp 74 is.
Referring back to FIG. 1, the analog-to-digital converting unit 732 is coupled electrically to the detector 72 for receiving the second detecting signal and the third detecting signal therefrom, and further receives a first burst signal (i.e., a DC voltage signal) from an external source. The analog-to-digital converting unit 732 converts the second detecting signal, the third detecting signal and the first burst signal respectively into corresponding digital values, namely a second detecting value, a third detecting value, and a first burst value.
The oscillator unit 733 generates an oscillating signal having a frequency larger than that of the drive signal.
The processing unit 734 is coupled electrically to the detector 72 for receiving the first detecting signal therefrom, and to the analog-to-digital converting unit 732 for receiving the second detecting value and the third detecting value therefrom. The processing unit 734 records a first calculation value, a second calculation value, a third calculation value, a current-setting value, and a voltage-setting value.
The first, second and third calculation values are defined by the following relations:
            N      1        =                  T        drive                    T        osc                        N      2        =                  T        duty                    T        osc                        N      3        =                  T        overlap                    T        osc            
wherein N1 denotes the first calculation value, N2 denotes the second calculation value, N3 denotes the third calculation value, Tdrive denotes the period of the drive signal, Tduty denotes the duration of the positive pulse or the negative pulse of the drive signal, Toverlap denotes the discharge duration to release energy stored by the primary winding 711, and Tosc denotes a period of the oscillating signal. The first to third calculation values and the oscillating signal are used to configure the waveform of the drive signal.
The first calculation value N1 has a preset value. The processing unit 734 gradually adjusts the first calculation value N1 from the preset value according to the first detecting signal received from the detector 72, such that a phase difference between the drive signal and the tank current is zero. At this time, the step-up transformer 71 is driven near the resonance frequency. Detailed description relating to the adjustment of the first calculation value N1 will be provided in the following paragraph.
The processing unit 734 determines voltage level of the first detecting signal upon switching of the third switch 763 of the switching unit 731 from the non-conducting state to the conducting state. When the first detecting signal is at a high voltage level, which indicates that the phase of the drive signal leads the phase of the tank current, the processing unit 734 increases the first calculation value N1 so as to delay the phase of the drive signal. On the other hand, when the first detecting signal is at a low voltage level, which indicates that the phase of the drive signal lags the phase of the tank current, the processing unit 734 reduces the first calculation value N1 so as to advance the phase of the drive signal.
The current-setting value is determined by the user. The processing unit 734 adjusts the second calculation value N2 and the third calculation value N3 according to a first difference between the second detecting value and the current-setting value as determined by the processing unit 734, so as to make the current flowing through the discharge lamp 74 correspond to the current-setting value. When the first difference indicates that the second detecting value is smaller than the current-setting value, the second calculation value N2 and the third calculation value N3 are increased by the processing unit 734. On the other hand, when the first difference indicates that the second detecting value is larger than the current-setting value, the second calculation value N2 and the third calculation value N3 are decreased by the processing unit 734.
The voltage-setting value is also determined by the user. The processing unit 734 determines whether the voltage of the secondary winding 712 of the step-up transformer 71 is normal by determining a second difference between the third detecting value and the voltage-setting value. When the second difference indicates that the third detecting value is greater than the voltage-setting value, which indicates that the voltage of the secondary winding 712 is too large, a warning signal is outputted by the processing unit 734 so as to protect the drive circuit and the discharge lamp 74.
The burst unit 735 is coupled electrically to the oscillator unit 733 for receiving the oscillating signal therefrom, to the analog-to-digital converting unit 732 for receiving the first burst value, and to the processing unit 734 for receiving the warning signal therefrom. The burst unit 735 further receives a second burst signal and a select signal from an external source. Frequency of the second burst signal is smaller than that of the drive signal, and timing of the high voltage level (or low voltage level) of the second burst signal is adjustable. The burst unit 735 conducts frequency division of the oscillating signal so as to generate a third burst signal, whose timing of high voltage level (or low voltage level) corresponds to that of the first burst value, and whose frequency is smaller than that of the drive signal. The burst unit 735 further outputs one of the second and third burst signals as a burst control signal according to the select signal. The burst unit 735 stops operating upon receipt of the warning signal.
The waveform generating unit 736 is coupled electrically to the oscillator unit 733 for receiving the oscillating signal therefrom, to the processing unit 734 for receiving the first to third calculation values N1, N2, N3, and the warning signal therefrom, and to the burst unit 735 for receiving the burst control signal therefrom. The waveform generating unit 736 configures the waveforms of the control sub-signals for the first to fourth switches 761˜764 of the switching unit 731, such as the waveforms 811˜814 shown in FIG. 3, according to the first to third calculation values N1, N2, N3 by counting the oscillating signal. The waveform generating unit 736 outputs the control signal, including the set of control sub-signals, to the switching unit 731 when the burst control signal is at one of a high voltage level and a low voltage level, and does not output the control signal to the switching unit 731 when the burst control signal is at the other one of the high voltage level and the low voltage level. The waveform generating unit 736 stops operating upon receipt of the warning signal.
As shown in FIG. 1, the burst control signal outputted by the burst unit 735 and the current-setting value recorded by the processing unit 734 cooperate to adjust the average current flowing through the discharge lamp 74 so as to adjust the brightness of light provided by the discharge lamp 74, thereby achieving light adjustment of the discharge lamp 74.
It should be noted herein that the processing unit 734 can also gradually adjust the first calculation value N1 according to the first detecting signal such that the phase difference between the drive signal and the tank current can be non-zero (detailed description of which will be provided in the following paragraph). At this time, the step-up transformer 71 is driven near, below, or above the resonance frequency.
In order to permit the phase difference between the drive signal and the tank current to be non-zero, the processing unit 734 further records a phase-setting value that is determined by the user, and further receives the oscillating signal from the oscillator unit 733 (connection between the oscillating unit 733 and the processing unit 734 is not shown in FIG. 1). The processing unit 734 delays the timing of determining the voltage level of the first detecting signal with reference to the phase-setting value by counting the oscillating signal. In particular, the timing of determining the voltage level of the first detecting signal is delayed by a duration of the phase-setting value multiplied by the period of the oscillating signal Tosc.
Referring to FIG. 4, waveform 821 represents the control sub-signal for the third switch 763 of the switching unit 731, and waveform 822 represents the first detecting signal. When the phase-setting value is smaller than the first calculation value N1, the phase difference between the drive signal and the tank current is less than zero. The step-up transformer 71 is driven at a frequency above the resonance frequency.
Referring to FIG. 5, waveform 831 represents the control sub-signal for the third switch 763 of the switching unit 731, and waveform 832 represents the first detecting signal. When the phase-setting value is greater than the first calculation value N1, the phase difference between the drive signal and the tank current is greater than zero. The step-up transformer 71 is driven at a frequency below the resonance frequency.
When the phase-setting value is equal to the first calculation value, the phase difference between the drive signal and the tank current is zero. The step-up transformer 71 is driven near the resonance frequency.
The conventional drive circuit automatically adjusts the frequency of the drive signal according to the phase of the tank current, such that the frequency of the drive signal changes with variations of the resonance frequency (e.g., caused by variations in the stray capacitance around the discharge lamp 74), so as to reduce efficiency differences among different conventional drive circuits during mass production.
However, since the waveform of the drive signal is configured by a digital control method in the conventional drive circuit, the smallest variation gradient in Tduty is Tosc. When Tduty changes, since the variation thereof is not continuous, but in steps of multiples of Tosc, the brightness of the light provided by the discharge lamp 74 changes abruptly (discontinuous), resulting in flashing of the light provided by the discharge lamp 74.
Moreover, since Tduty is adjusted by first converting the second detecting signal that corresponds to the current magnitude of the current flowing through the discharge lamp 74 into the corresponding digital second detecting value, and then by determining the first difference between the second detecting value and the current-setting value, and since a time lag exists between the second detecting value and the second detecting signal due to analog-to-digital conversion, adjustment of Tduty by the conventional drive circuit is not in real time, which easily results in malfunctioning of the conventional drive circuit or instability in the brightness of the light provided by the discharge lamp 74.